Highly accurate breath test system

ABSTRACT

The invention relates to a system for breath test of a person. It includes a sensor unit configured to sense the presence/concentration of a volatile substance, e.g. alcohol, present in air flowing through a predefined inlet area and generating a signal corresponding to the concentration of said substance. An analyzer determines the concentration of said substance in the breath of said person. It comprises means for the temporary interruption of said air flow at a point in time coinciding with the detection of a breath. It also relates to a method comprising interrupting the flow through said predefined area for a predetermined period of time, and detecting the concentration of said substance during said interruption.

This invention is concerned with a system for sensing thepresence/concentration of substances, such as ethyl alcohol, within theexpired breath of a person.

BACKGROUND OF THE INVENTION

Breath alcohol concentration (BrAC) is related to blood alcoholconcentration (BAC) by the approximate relationBrAC[mg/1]=0.5*BAC[mg/g]. Other substances will have differentcoefficients.

Supervised breath tests according to the state of the art are beingperformed by the police in order to prevent drunk driving. For the samepurpose, unsupervised tests using alcolocks in vehicles are also beingincreasingly used. Sensor technologies include catalytic semiconductors,fuel cells and infrared spectroscopy. Performance with respect toaccuracy, specificity, environmental immunity, and response time, ishighly variable between different devices available on the market.Devices for breath test include sensor elements providing a signalrepresenting BrAC after taking a deep breath, and emptying the airwaysvia a tight-fitting mouthpiece, which for hygienic reasons has to be aseparate, disposable item. In order to ensure a correct determination,the test person is required to deliver a forced expiration at almostfull vital capacity. This requires substantial time and effort,especially for persons with limited capacity. The handling ofmouthpieces is time-consuming and represents an undesired source oferror due to water condensation. The accuracy of the determinationrepresents an increasing challenge, especially when the determination isrelated to legal limits. Highly accurate breath analyzers for evidentialpurposes are commercially available, but they are expensive. There is astrong market pull for mass produced devices capable of accurate andreliable breath testing at low cost, and minimum effort for the personto be tested.

The basic techniques of breath analysis were developed during the secondhalf of the 20th century. More recently, a movement towards lessobtrusive means for breath test has been noted. Olsson et al (WO98/20346) disclosed a system solution in which accurate measurementscould be performed without a mouthpiece using water vapor as a tracergas. Lopez (U.S. Pat. No. 5,458,853) reported another approach, usingultrasound to correct for the dependence on distance between the deviceand the user's mouth. Hök et al (GB 2431470) disclosed a system solutionusing carbon dioxide (CO₂) as a tracer gas, combined with a simplealgorithm for correction of a diluted breath sample. Still anotherapproach was reported by Lambert et al (SAE World Congress Apr. 3-6,2006). The air within a vehicle cabin was monitored, and an alcoholadsorbing material was used to accumulate the sample to enhanceresolution. Again, CO₂ was used as a tracer gas.

SUMMARY OF THE INVENTION

The object of the present invention is to increase the accuracy ofbreath test systems. The invention is based on a few critical elementswhich in combination will provide the necessary characteristics. First,there is a sensor unit providing a signal corresponding to the alcoholconcentration of air flowing through a predefined inlet area, by whichis meant one or several openings allowing air to be continuously flowingfrom the inlet area to the sensor unit. Second, an analyzer is includedfor the determination of breath alcohol concentration of said personbased on the sensor signal. A third element is means for controlling theinterruption of the air flow at a point in time coinciding with thedetection of a breath sample, allowing the sensor unit to be purgedafter a short period of time upon completion of the determination. Inone embodiment of the system according to the invention, means forgenerating or assisting air flow, e.g. a fan or pump is also included inthe system. In this embodiment, flow interruption is accomplished byactively turning off the fan upon detection of a breath, and automaticpurging after the determination. In another embodiment without a fan orpump, flow interruption is accomplished by means of a flap valve. Theseembodiments can, of course, also be used in combination.

The combined function of the basic elements is necessary and sufficientfor obtaining the required accuracy. One improvement of the presentinvention is that the sampled air will be trapped in the sensor unit byinterrupting the air flow. The breath analysis will therefore beperformed at higher concentration than would otherwise be the case. Thisalso leads to improved accuracy.

By interrupting the air flow, it is possible to prolong the measurementtime, and perform signal averaging. Random errors are then reduced by afactor √(T_(av)/T_(s)), where T_(av) is the averaging time, and T_(s) isthe time between two signal samples. For example, with T_(s)=0.2seconds, and T_(av)=2 seconds, the accuracy will be improved by√10=3.16.

The benefits of interrupting the air flow will be even more dramaticwith respect to flow-related systematic errors, which are manifested byfalse readings. Influence of air flow exists both as a direct effect,and indirectly via temperature gradients. These errors will be highlydepending on the actual components used, and are significant especiallyat low concentrations.

The flow interruption should be only temporary, during determination ofbreath alcohol concentration or measurement of background concentration.Otherwise air flow through the measurement cell should be maintainedwhenever the system is active, allowing variations in ambient gasconcentrations to be continuously monitored. Interruption also includestemporary reduction of the air flow to a significantly lower magnitude,without necessarily stopping it completely.

The present invention allows breath tests to be performed in a varietyof circumstances which have hitherto been inaccessible. The improvedaccuracy, usability and possibility of vehicle integration may be animportant step towards preventing drunk driving on a much larger scalethan with products available at the present. This is believed necessaryto reduce the high mortality of alcohol related traffic accidents. Otherpromising application areas are sobriety control of staff with criticaltasks, and of audience arriving at an arena. It may also be used invarious self test scenarios, e.g. in the treatment of alcoholics. Thepossibility of unobtrusive breath tests is expected to become importantfor diagnostic purposes in emergency medicine. For this purpose, a largenumber of volatile substances are of interest in addition to ethylalcohol.

To meet the object the invention in a first aspect provides a system forbreath test of a person as defined in claim 1. It includes a sensor unitconfigured to sense the presence/concentration of a volatile substance,present in air flowing through a predefined inlet area and generating asignal corresponding to the concentration of said substance, an analyzerfor the determination of the concentration of said substance in thebreath of said person, the determination being based on said signalcorresponding to the substance concentration, and means for thetemporary interruption of said air flow at a point in time coincidingwith the detection of a breath.

In a second aspect it provides a method of performing a breath test of aperson defined in claim 12. It comprises the steps of providing a testsystem comprising a sensor configured to sense thepresence/concentration, of a volatile substance, present in air from thebreath of said test person flowing through a predefined inlet area (4)and generating a signal corresponding to the concentration of saidsubstance, interrupting the flow through said predefined area for alimited period of time, and detecting the concentration of saidsubstance during said interruption

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described below with reference to the drawingsin which

FIG. 1 shows a schematic drawing of the system according to oneembodiment.

FIG. 2 shows a flow graph of the system function.

FIG. 3 shows the time sequence of typical breath test performed with thesystem according to the invention.

DETAILED DESCRIPTION

FIG. 1 shows a schematic drawing (not to scale) of one embodiment of thesystem 1 according to the invention.

It comprises a sensor unit 5 including a compartment C which forms apassage for breath air that is to be analyzed, a source 6 of IR light,i.e. an IR emitter, and a first sensor 8 capable of detecting a volatilesubstance, e.g. ethanol, a second sensor 7 capable of detecting e.g.CO₂, and suitably a fan or a pump 9, driven by a motor.

The system suitably also comprises a presence detector 2 for detectingthat a test person is in the vicinity of the system, and also preferablyan audiovisual unit 3 comprising a display unit 3 b and a loudspeaker 3a.

The system also comprises an analyzer 10, which preferably includes ageneral purpose digital microcontroller with capacity to execute signalalgorithms, and means 14 for bidirectional control of current drivepulses to the motor driving the fan or pump.

A test person 13 is shown positioned in the vicinity of an inlet area 4of the sensor unit 5, equipped with a sensor element 8 generating asignal corresponding to the ethyl alcohol concentration of the airflowing through the inlet area 4. Air flow generating means through thesensor unit 5 is provided by a fan or pump 9. The inlet area 4constitutes one or several openings, into which air can be freelyflowing, driven by the fan 9. Preferably, a particle filter 11 e.g. madefrom porous material is included in the inlet area 4. This preventsparticles and aerosols from contaminating the sensor unit 5 while notimpeding the air flow to any significant degree. There is also an outletarea 12 from which the air flow is returned into the ambient. In oneembodiment of the invention, the outlet area includes a flap valveindicated in FIG. 1 by two hinged thin walls, allowing air flow in onedirection only. To remain open, it is necessary to maintain a horizontalpressure gradient (direction referred to FIG. 1) across the thin walls.If such a gradient is too small, absent or reversed, the hinged wallwill fall back into a vertical position closing the outlet area 12. Theexact closure point of the flap valve is depending on the properties ofthe hinges and walls, and may be adjusted to any particular requirement.There are also options to a flap valve, such as an electromagneticallycontrolled valve.

When the person 13 is directing expiratory air towards the inlet area 4from a distance not exceeding 50 cm, the air flowing through the sensorunit 5 will consist of a mixture of ambient and expiratory air from theperson 13.

The present system is capable of interaction with the person 13 during ashort moment of time. The apparatus for this includes means forregistration 2 of the presence of a person 13 at a position in thevicinity of the inlet area 4, and an audiovisual unit 3. Theimplementation of the means of registration 2 is highly depending on theactual application and could include a microswitch indicating dooropening/closure, microphone, camera, contactless detector usingultrasound or infrared radiation, force sensor responding to the weightof the person. It may include means for identification of the person byvoice control, image analysis, bar-code reading, or biometric analysis.The audiovisual unit 3 preferably includes a loudspeaker 3 a and adisplay 3 b. The loudspeaker 3 a may generate artificial speech orsymbolic sound tracks, and the display 3 b may convey text, images,icons or other symbols.

Preferably, the audiovisual unit 3 is located in close vicinity to theinlet area 4, in order to direct the person's 13 attention to this area.It is capable of calling for the immediate attention of the person 13upon presence registration or at some later instant. It is also capableof conveying an instruction, even a detailed one, in the case that theperson 13 may need one.

As a consequence of mixing between ambient and expiratory air, thesignal generated by the sensor element 8 will be diminished by a factorcorresponding to the dilution of the expiratory air. Therefore, anothersensor element 7 is included in addition to the element 8, for measuringthe concentration of a tracer gas, e.g. carbon dioxide (CO₂) or watervapor. Since the tracer gas concentration is approximately constant whenleaving the airways, it is possible to obtain a fair approximation ofthe degree of dilution of the air entering the sensor unit 5. Anotheroption for a tracer signal besides CO₂ and H₂O is temperature. Thetemperature of expiratory air is almost the same as body temperature asit leaves the mouth or nose but will get closer to ambient upon mixing.

The sensor elements 7 and 8 constitute the receiver ends of ameasurement cell for infrared (IR) transmission measurement. From aninfrared emitter 6, preferably a blackbody radiating element, a beam ofbroadband infrared radiation is illuminating the cell, and eventuallyafter multiple reflections it will reach the elements 7, and 8.Preferably, the emitter 6 is modulated at a frequency, e.g. 5 Hz, abovethe frequency band of typical signals. Each of the sensor elements 7 and8 include thermopile detectors of infrared radiation with bandpassinterference filters tuned to the absorption peak of the substance to bedetected. The element 8 includes a filter with the pass band within theinterval 9.1-9.9 μm for ethyl alcohol, and the element 7 the filter inthe interval 4.2-4.3 μm in the case of CO₂ as tracer gas. Water vapor,an alternative tracer gas, has strong absorption in the wavelengthintervals 2.5-2.8 μm and 5.7-6.9 μm. Other combinations of gases andfilter characteristics are possible. Acetone, acetaldehyde, methylalcohol, carbon monoxide, methane, ethane, propane, pentane, hexane,heptane, octane, isoprene, ammonia, hydrogen sulfide, methyl mercaptan,ethyl acetate, dimethyl ether, diethyl ether, benzene, toluene, methylethyl ketone, and methyl isobutyl ketone are examples of volatilesubstances that may be of interest from a diagnostic or toxicologicalperspective.

The optical path from the IR emitter 6 to the detectors 7, and 8 maydepend on the concentration range and the absorption coefficients of theactual substances. CO₂ has strong absorption and high concentration inexpiratory air which calls for a short optical path, 10-25 mm. Foralcohol detection below the legal concentration limits, path lengths ofmore than 0.5 m may be necessary. By folding the optical path usingmultiple reflections, the length/width/height of the sensor unit 5 canstill be kept smaller than 70/30/15 mm.

The sensor unit 5 responds almost instantaneously, i e within a fractionof a second, to concentration variations occurring at the inlet area 4.This is partly due to the small distance between the inlet area 4 andthe sensor unit 5, typically 10-20 mm, its small inner volume, typically20-60 ml, and the air volume flow, typically 100-200 ml/sec, generatedby the fan 9, and the air flow velocity generated by the fan 9. It isalso due to the relatively fast modulation frequency of the infraredemitter. The signal information extracted from the sensor elements 7 and8 is represented as the amplitude of the modulation frequency.

In order to meet requirements on electromagnetic emission and immunity,the system according to the invention includes capacitive and inductiveelectronic elements for protective purposes. In addition, the elements 7and 8 and their associated analog input stages are preferably equippedwith differential preamplifiers in order to suppress the influence ofcommon mode interference.

The signals from the sensor elements 7, 8 are brought to an analyzer 10,which preferably includes a general purpose digital microcontroller withcapacity to execute signal algorithms, and also controlling theaudiovisual unit 3, IR emitter 6, fan 9. Signal conversion betweendifferent formats, including analog signals, can be managed by themicrocontroller 10, which will also be capable of communicating withexternal units, e.g. an actuator unit for taking action or counteractiondepending on the result of the breath test. Electric power for thesystem 1 can either be obtained from a battery or from an external powersource. The system 1 can be designed as a stand-alone handheld unit, oras an integrated part of other inventories, e.g. a vehicle compartmentor entrance of building or workplace. Preferably, the inlet area 4includes means for protection of the sensor unit 5, e.g. a lid which isclosed when the system 1 is inactive. The flap valve 12 is capable offulfilling this function.

Preferably, the moving parts of the fan 9 have a small mass, typicallyless than 1.5 gram, in order to have minimum start and stopping time.The fan 9 preferably also includes a brushless DC motor, and means 14for bidirectional control of current drive pulses to the motor, makinguse of the fact that the electromagnetic effect is reversible betweenmotor and generator function, designated in FIG. 1 by the bidirectionalarrow between the fan 9 and control circuit 14, available from severalsuppliers, e.g. Texas Instruments Inc., USA. By this control function itis possible, both to start/stop the fan 9 very quickly, and to run it atdifferent speeds. Its feedback loop may also include a flow sensor,measuring the actual air flow. Start and stopping times can be kept at aminimum by the control and drive circuit 14.

In the off mode, the fan 9 represents a significant flow constriction,which effectively traps the air inside the sensor unit 5. The crosssection area of the fan 9 allowing free passage of air is considerablytypically less than a fourth of the inlet area 4. This constrictionconstitutes a flow resistance preventing undesired venting of the sensorunit 5 during the time of measurement.

The system according to the invention is preferably confined in a box tobe wall-mounted in such a way that the means for registration 2,audiovisual unit 3, and inlet area 4, are located on one side of the boxand thereby accessible through a hole in the wall.

The air flow control means 10, 14 may also be used for other purposesthan improving accuracy. It may also be used during startup of thesystem for improving the stability of the sensor unit (5), andminimizing the startup time. Another use is to monitor long-termdegradation of bearings or other sensitive parts.

FIG. 2 shows a flow diagram of the system function according to theinvention. The system is started or initiated either manually orautomatically, by some external control signal. In the case of avehicle, the start signal could be unlocking of the vehicle doors. Theinitiating phase preferably involves some self-testing functions of thesystem, to make sure that no functional errors have occurred since theprevious test occasion. The fan 9 is automatically started, and isrunning at full or reduced speed until it receives a command tointerrupt the flow or run at a different speed. The initiating phasecould also include preheating of sensitive components and stabilizationof signals.

When the system is ready for test it will remain in a standby conditionuntil the presence of a person within the predefined position isdetected. As previously described, detection may or may not involveidentification of the person, and could require two-way communicationbetween the person and the system. After or during the presencedetection step, the system will call for the person's attention bycoordinated flashing light, distinctive and directional sound combinedwith specific symbol or icon representing the breath test.

An experienced person is then expected to direct expiratory air towardsthe sensor inlet area, whereas an unexperienced person may require amore or less detailed instruction on how to proceed. Example ofinstruction provided verbally or as a text message: “Take a deep breath,lean over, open your mouth wide and exhale gently.” Alternatively,instructions are provided by still or moving images, graphic symbols orother means. If the criteria for breath detection are not fulfilledafter one round of instruction, repeated instructions may be deliveredat increasing level of detail.

The criteria for breath detection preferably involve tracer gasdetection as previously described. In the case of CO₂ as tracer gas, asimple criterion is reaching a threshold CO₂ concentration of e.g. 2500ppm (parts per million), which corresponds to a dilution factor of 20(alveolar CO₂ concentration being approximately 5 vol %, or 50 000 ppm).Additional criteria could be related to the time derivative of the CO₂signal. The simultaneously measured alcohol concentration will in thiscase have to be multiplied with 20 in order to obtain an estimatedbreath alcohol concentration. The criteria for breath detection shouldalso include correction for background CO₂ concentration, which istypically 400-600 ppm in normal environments. A mathematical expressionor algorithm will normally be adequate for defining the criteria, usingsettable parameters to adapt for variations between differentconditions. Such an algorithm can be implemented for execution in realtime using standard microcontrollers.

Upon detection of a breath, a command is sent to the fan 9 to interruptthe air flow. The air within the sensor unit will then become trapped,and prolonged measurement may be performed at zero air velocity, whichwill basically eliminate flow related errors, and allow signal averagingresulting a reduction also of random error, such as noise. The reductionwill apply to the determinations of both the substance and the tracer.

In one embodiment of the invention, in the absence of a fan or pump 9,there is no electronic control of the flow interruption. In thisembodiment, the air flow is driven by the exhaling person, creating apressure gradient across the hinged walls of the flap valve 12, andallowing air to pass the outlet area. When the person's exhalation isdecreasing or stopping, the flap valve will close, trapping the exhaledair in the sensor unit 5. The exact time of closure is depending on theelasticity of the hinges and walls of the flap valve, and may be adaptedto coincide with the detection of a breath, eventually with some delay.

The level of dilution is a measure of the signal quality. Highconcentration (small dilution factor) provides high confidence of thedetermination, whereas the influence of interfering factors, such asother nearby persons, will increase with degree of dilution. Preferably,the result of the breath test is presented not only as a concentrationbut also in terms of an estimated error depending on the dilutionfactor.

Breath detection may in some applications override the presencedetection as symbolized in FIG. 2 by the dotted line short-cutting boththe ‘attention’ and ‘instruction’ sequences. Another way of expressionis to include the tracer gas detection into the ‘means of registration’.

Determination of BrAC is performed by another algorithm based on thecorrelation between the signals from the sensor elements 7 and 8. Whenthe sensor unit 5 is receiving expired air from a person, both sensorelements exhibit concentration peaks which occur almost simultaneously.An average BrAC value is obtained by multiplying a number of measuredalcohol concentrations by their respective dilution factors. By signalaveraging, the effect of noise and interference is reduced. A small timedifference between the CO₂ and the alcohol signals due to differencescaused by the anatomic dead space or by the design of the sensor unit 5is also possible to accommodate in the algorithm.

The completion and result of a breath test defined by fulfillment of thecriteria for breath detection, is preferably communicated to the person,e.g. using the audiovisual unit 3. The fan 9 is preferably commanded torestart after completed determination in order to purge the sensor unit,preferably at full speed in order to minimize the time before the systemis ready for a new breath test. The restart takes place within a limitedperiod of time, such as 1-10 seconds, preferably 1-5 seconds. If no fanor pump 9 is included, the system 1 if handheld may be manually purgedby moving it swiftly in the horizontal right to left direction referringto FIG. 1, thereby creating a pressure gradient across the flap valve12, and allowing fresh ambient air to enter the sensor unit 5.

Basically the same flow diagram applies to the embodiment without a fanor pump, only that purging of the sensor unit 5 is performed manually.

In the flow diagram of FIG. 2, the further steps taken after terminationof the actual breath test are not included, since they may be highlydepending on the actual application of the breath test. Such steps mayinvolve rule-based decision for controlling action or counteractionbased on the determination, e.g. enabling/disabling functions of avehicle or locking/unlocking of door.

FIG. 3 shows a time sequence of a typical breath test performed with thesystem according to the invention. The signals represented are from topto bottom: ‘F’ control signal to the fan 9, in which ‘high’ represents‘on’ and ‘low’ represents ‘off’, ‘T’ tracer gas, ‘A’ alcohol. Thetime-scale represents a typical sequence of an experienced test personreacting to a request, at time zero, to deliver a breath sample byexhaling towards the inlet area 4. After approximately one second, atracer signal above a threshold value is noted, and the fan 9 receives acommand from the control circuit 14 to interrupt the air flow. After ashort delay, the signals of the tracer and the alcohol channels reach aplateau, from which an accurate determination of dilution and BrAC ismade, using signal averaging. After completion of the determination, anew command for restart is sent to the fan 9. Upon venting of the sensorunit 5, the signals of both the tracer and alcohol channels return tothe original level.

Basically the same time sequence applies to the embodiment without a fanor pump, only that purging of the sensor unit 5 is performed manually.

1. A system for breath test of a person including a sensor unitcomprising a first sensor element configured to detect the presenceand/or concentration of a volatile substance, present in air from abreath flowing through a predefined inlet area and to generate a signalcorresponding to the concentration of said substance; a second sensorelement configured to detect the presence of a tracer gas in saidflowing air; an analyzer configured to determine the concentration ofsaid substance in the breath of said person, the determination beingbased on said signal corresponding to the substance concentration; meansfor generating and controlling said air flow, including a fan or a pump,said means being adapted to interrupt said flow upon the detection of abreath, wherein the criteria for breath detection includes tracer gasdetection.
 2. The system according to claim 1, further comprising ameans for registration of the presence of a person at a position in thevicinity of the inlet area, and an audiovisual unit.
 3. The systemaccording to claim 1, wherein said means for generating and controllingsaid air flow is configured to restart said flow after a limited periodof time, suitably 1-10 seconds, more preferred 1-5 seconds.
 4. Thesystem according to claim 1, further comprising a valve for execution ofsaid interruption.
 5. The system according to claim 1, wherein saidsensor unit includes means for breath detection based on thedetermination of a tracer substance, e.g. carbon dioxide, or of watervapor, or temperature.
 6. The system according to claim 1, wherein saidflow generating means includes a brushless DC motor, and means forbidirectional control of current drive pulses to said motor.
 7. Thesystem according to claim 1, wherein the free cross section area of saidflow generating means is less than a fourth of said inlet area.
 8. Thesystem according to claim 1, wherein the mass of moving parts of saidflow generating means is less than 1.5 grams.
 9. The system according toclaim 1, further comprising control means for controlling said means forgenerating or assisting said air flow, including feedback from actualdrive elements, e.g. a brushless DC motor, or a flow sensor.
 10. Thesystem according claim 1, wherein the volatile substance is acetone,acetaldehyde, methanol, ethanol, carbon monoxide, methane, ethane,propane, pentane, hexane, heptane, octane, isoprene, ammonia, hydrogensulfide, methyl mercaptan, ethyl acetate, dimethyl ether, diethyl ether,benzene, toluene, methyl ethyl ketone, or methyl isobutyl ketone or acombination thereof.
 11. The system according to claim 1, wherein thedistance between the inlet area and the sensor unit is typically 10-20mm.
 12. The system according to claim 1, wherein the inner volume of thesensor unit is typically 20-60 ml.
 13. The system according to claim 1,wherein air volume flow generated by the fan is typically 100-200ml/sec.
 14. The system according to claim 1, which is confined in a boxadapted to be wall-mounted in such a way that the means forregistration, audiovisual unit, and inlet area, are located on one sideof the box and thereby accessible through a hole in the wall.
 15. Themethod of performing a breath test of a person, comprising the steps of:detecting, using a first sensor element, the presence and/orconcentration of a volatile substance present in air from a breathflowing through a predefined inlet area; generating a signalcorresponding to the concentration of said substance; detecting, using asecond sensor element, presence of a tracer gas in said flowing air;determining, using an analyzer, a concentration of said substance in thebreath of said person, based on said signal corresponding to thesubstance concentration; controlling said air flow using a fan or a pumpadapted to interrupt said air flow upon the detection of a breath;interrupting the air flow through said predefined area for a limitedperiod of time; and detecting the concentration of said substance duringsaid interruption.